1. Field of the Invention
The present invention relates to an autotensioner used for applying an appropriate tension to a timing belt of an automotive engine.
2. Description of the Related Art
Drive units which use a timing belt or poly-V belt for driving a camshaft of an engine in synchronous timing with the crank shaft are widely known.
The timing belt has an inner peripheral face formed with teeth, so that these teeth are engaged with the teeth formed on the outer peripheral face of the drive and follower pulleys.
With this type of timing belt, it is necessary to apply an appropriate tension to prevent so-called "tooth-jump" with creep of the teeth on the inner peripheral face of the timing belt relative to the teeth on the outer peripheral face of the drive pulley and follower pulley due to e.g. temperature changes.
It is therefore common to use an autotensioner which presses a pulley against the outer peripheral face of the timing belt under resilient spring force to thereby maintain a constant tension in the timing belt irrespective of temperature changes.
An autotensioner used for this purpose must achieve the following functions (1) and (2):
(1) the autotensioner must be able to press resiliently against the outer peripheral face of the timing belt so as to apply an appropriate tension to the timing belt , and
(2) when the tension in part of the timing belt at the portion pressed by the pulley rises suddenly, the autotensioner must be able to press strongly against that portion, without retreating immediately, to thus prevent the tension in the other parts of the belt being excessively reduced.
An autotensioner is therefore provided with a spring to ensure the above function (1), and a damper unit to ensure function (2).
For the damper unit, there is the so-called one way configured unit such as disclosed in Japanese Patent First Publication KOKAI No. S63-180759, having a damper function in both directions. However, in order to further decrease tension fluctuations in the timing belt, it is preferable to have a so-called two way configured unit with a damping function in one direction only, so that the pulley pressing against the timing belt can be rapidly displaced in the direction of pressure and gradually displaced in the opposite direction.
Japanese Utility Model First Publication KOKAI No. 60-23355 discloses a construction as shown in FIGS. 1 and 2, for an autotensioner incorporating such a two way configured damper unit.
FIG. 1 shows a timing belt drive unit with an autotensioner 4 fitted in it. Rotation of an engine crank shaft is transmitted from a drive pulley 1 to a timing belt 2, to thereby rotate follower pulleys 3 and cam shafts (not shown in the figure) fixed to respective ends of the follower pulleys 3. A follower pulley 3a is provided for driving auxiliary machinery of a water pump and the like.
The autotensioner 4 has a pulley 5 which is resiliently pressed against a portion of the timing belt 2 at a central part between the drive pulley 1 and the follower pulley 3, to thereby apply an appropriate tension to the timing belt 2.
The pulley 5 is pivotally supported by a second shaft (not shown in the figure) at a central portion of a swinging member 6.
The swinging member 6 is also referred to as pivotal member or rocking member.
The swinging member 6 is pivotally supported on a fixed portion or front face of the engine cylinder block (not shown) by means of a first shaft or pivot shaft 7.
Moreover, a damper unit 9 as shown in detail in FIG. 2, is provided between a bearing portion 8 on a tip end of the swinging member 6, and the front face of the cylinder block (not shown).
The damper unit 9 is fixed, for example to the front face of the cylinder block (not shown), at a portion separate from the swinging member 6, and exerts a resilient force in a direction of extension in overall length, based on the resilient force of a compression spring 11 housed in a cylinder 10, so that the pulley 5 is resiliently pressed towards the timing belt 2, under the resilient force of the compression spring 11.
Moreover, due to the actions of a viscous liquid 12 sealed inside the cylinder 10, and of a check valve 13, the damper unit 9 extends rapidly but contracts gradually. Accordingly, the autotensioner 4 satisfies the beforementioned functions (1), (2).
With the autotensioner 4 shown in FIGS. 1 and 2, since the resilient force for pressing the pulley 5 against the timing belt 2 comes from the compression spring 11 housed inside the damper unit 9, the damper unit 9 is relatively large. That is to say, in order to apply a sufficient tension to the timing belt 2, the resilient force of the compression spring 11 must be large, and since in general this resilient force must be maintained at around 10 to 20 kgf, it is difficult to avoid having a large compression spring 11 (in length and diameter). The damper unit 9 in which the compression spring 11 is housed thus becomes large (in length and diameter), so that it becomes difficult to install an autotensioner 4 incorporating such a large damper unit 9 in a limited installation space, for example at the front face of the cylinder block.
The damper unit 9 of the conventional construction shown in FIGS. 1 and 2 also lacks universal application, requiring a large variety of damper units 9 to suit autotensioners for different kinds of engines, resulting in an increase in manufacturing costs of the autotensioner 4, due to the complexity of component manufacture and component management. That is to say, not only does the set value for the tension of the timing belt 2 change with engine displacement, but also there are subtle differences, due for example to the engine type (whether OHC or DOHC). Accordingly, it is necessary to change the resilient force of the compression spring 11 to suit the set value. However, when the resilient force is changed, the length and diameter of the compression spring 11 become different. Therefore, the size of the damper unit 9 in which the compression spring 11 is housed is variously changed depending on the different set values, causing an increase in the abovementioned manufacturing costs.
To address this problem Japanese Utility Model First Publication KOKAI No. 06-47757 discloses an autotensioner such as shown in FIGS. 3 to 5 having a construction wherein a common damper unit is possible for configurations having different set values for the tension, and also the damper unit can be of small size. A damper unit such as shown in FIG. 6, is fitted to this conventional autotensioner.
The damper unit shown in FIG. 6 is a specific device based on the damper unit disclosed in the above publication KOKAI No. 06-47757. Since the autotensioner shown in FIGS. 3 to 5 and the damper unit shown in FIG. 6 are substantially the same in basic construction to the autotensioner and damper unit used in the tests during the stages to completion of the present invention, these will be briefly described in detail.
As shown in FIGS. 3 to 5, a fixed or stationary member 14 is fixed to a fixed portion, for example on the front face of the cylinder block, by means of a bolt (not shown) inserted into an attachment hole 15 in a central portion of the fixed member 14. A first shaft or fixed shaft 17 is inserted into an aperture 16 formed in an end portion (upper end portion in FIGS. 3 to 5) on the fixed member 14. A base end of the fixed shaft 17 is internally fixed to the aperture 16.
A swinging member 18 is pivotally supported by the fixed member 14. Specifically, a base end (lower end portion in FIGS. 3 to 5) of the swinging member 18 is supported so as to turn freely about the fixed shaft 17 by externally fitting a cylindrical portion 19 formed on the base end of the swinging member 18, around the fixed shaft 15 by way of a plain bearing 20. Furthermore, a bolt 21 passing through the fixed shaft 17 is screwed into a threaded hole formed in a fixed portion, such as on the front face of the cylinder block, thereby co-operating with the bolt (not shown) inserted into the attachment aperture 15, to prevent turning of the fixed member 14.
A second shaft or short-cylindrical protrusion 22 parallel with the first shaft or fixed shaft 17, is formed on a tip end (upper end in FIGS. 3 to 5) of the swinging member 18. A pulley 5 is rotatably supported around the protrusion 22 by a rolling bearing 23. More specifically, a bolt 24 is passed through a central aperture of an inner ring 25 of the rolling bearing 23, and through a washer 26 and then tightened with a nut 27, to thus retain the peripheral portion around the central aperture of the inner ring 25. With this location of the pulley 5, the fixed shaft 17 is located radially outwards from the outer peripheral face of the pulley 5. In other words, the first axis along which the fixed shaft is located is spaced apart from the second axis along which the pulley 5 is located, by a length longer than the radius of the pulley 5.
A coil portion 29 of a torsion coil spring 28 is located around the cylindrical portion 19 of the swinging member 18. One engaging portion 30a of the torsion coil spring 28 is engaged in a engaging aperture 31a formed in the fixed member 14, while another engaging portion 30b is inserted via a sleeve 32, into an engaging aperture 31b formed in the swinging member 18. Due to the torsion coil spring 28, a resilient force is applied to the swinging member 18 so as to turn it in a clockwise direction in FIG. 3, about the fixed shaft 17.
A damper unit 34 has a base end which is supported on a fixed arm or projection 33 provided on a part of the fixed member 14 at a location away from the fixed shaft 17.
Furthermore, the swinging member has a swinging arm 35 which is provided on a part of the swinging member 18 at a location away from the protrusion 22, so that a bearing or receiving block 37 is internally fixed in a cavity 36 formed in the swinging arm 35.
The damper unit 34 as shown in FIG. 6, has a cylinder 39 within which a viscous liquid 12 is contained, a plunger 38 which has a tip end abutted against an end face of the bearing block 37 and a piston 40 which is fitted inside the cylinder 39 so as to be freely movable in the axial direction (up/down direction in FIG. 6). A biasing spring or compression coil spring 41 is provided between the piston 40 and an internal end face of the cylinder 39 (lower end face in FIG. 6) so as to press the piston 40 in a direction out of the cylinder 39.
The plunger 38 has a base end face thereof (lower end face in FIG. 6) abutted against the piston 40. Accordingly, when the piston 40 is displaced (pushed upwards) due to the resilient force of the biasing spring 41, the amount of protrusion of the plunger 38 from the cylinder 39 increases.
An oil passage 42 is formed in a central portion of the piston 40, thus providing communication between both axial end faces of the piston 40. A lower end opening of the oil passage 42 is opened or closed by a bell 43 which is pressed against the opening under the resilient force of a compression spring 44, thus making up a ball valve type check valve 45. The check valve 45 closes when the piston 40 is displaced (lowered) against the resilient force of the biasing spring 41, and opens when the piston 40 is raised due to the resilient force of the biasing spring 41.
As shown in FIGS. 3 to 5, a stopper pin 46 is provided to immobilize swinging of the swinging member 18 about the fixed shaft 17 irrespective of the resilient force of the torsion coil spring 28, to thus facilitate the fitting of the timing belt 2 around the pulley 5. In this respect, the swinging member 18 is swung or pivoted against the resilient force of the torsion coil spring 28, to align a small hole 47 formed in the swinging member 18 and a small hole 48 formed in the fixed member 14, and the stopper pin 46 is then inserted through both holes 47, 48.
In this condition, the timing belt 2 can be easily fitted around the pulley 5, since the pulley 5 which is supported on the swinging member 18, is not displaced by the resilient force of the torsion coil spring 28. After fitting the timing belt 2, the stopper pin 46 is removed, so that the pulley 5 presses against the timing belt 2 under the resilient force of the torsion coil spring 28.
During operation of the autotensioner constructed as described above, the swinging member 18 is swung or pivoted by the resilient force of the coil spring 28, so that the pulley 5, rotatably mounted on the tip end portion of the swinging member 18, is resiliently pressed against the timing belt 2, thus restricting the movement of the swinging member 18, so that there is no further displacement of the swinging arm 35 located on the swinging member 18. As a result when the piston 40 of the damper unit 34 is displaced due to the resilient force of the biasing spring 41, so that the amount of protrusion of the plunger 38 from the cylinder 39 increases, the tip end of the plunger 38 is pressed against the bearing block 37 supported on the tip end portion of the swinging arm 35.
If the timing belt 2 slackens from this condition, the swinging member 18 is swung or pivoted in a clockwise direction in FIG. 3 about the fixed shaft 17 under the resilient force of the coil spring 28, so that the pulley 5 follows the movement of the timing belt 2. At this time, the displacement of the plunger 38 is slightly delayed, so that the tip end of the plunger 38 separates from the bearing block 37.
Therefore, when the timing belt 2 slackens, the swinging member 18 which is swung or pivoted in order that the pulley 5 follows the movement of the timing belt 2, receives absolutely no resistance from the damper unit 34. The pulley 5 can thus quickly follow the movement of the timing belt 2, thereby avoiding a drop in tension in the timing belt 2.
The plunger 38 moves out of the cylinder 39 under the resilient force of the biasing spring 41, slightly slower than the movement of the swinging member 18, until the tip end thereof bumps against the bearing block 37. When the plunger 38 is forced from the cylinder 39 in this way under the resilient force of the biasing spring 41, the check valve 45 inside the damper unit 34 opens, so that the piston 40 and plunger 38 are displaced comparatively quickly. There is thus only a very short time delay before the tip end of the plunger 38 bumps against the bearing block 37.
On the other hand, when the tension force in the timing belt 2 increases the swinging member 18 tends to rotate in a counterclockwise direction in FIG. 3 about the fixed shaft 17 against the resilient force of the torsion coil spring 28, so that the bearing block 37 is pressed against the tip end of the plunger 38. Therefore, the plunger 38 and the piston 40 must be pressed into the cylinder 39 against the resilient force of the biasing spring 41 and against the damper resistance in order to rotate the swinging member 18.
At this time, the ball 43 of the check valve 45 housed in the damper unit 34 presses against the opening of the oil passage 42 for communication between both sides of the piston 40, thus closing off the oil passage 42. The viscous liquid 12 on the lower side of the piston can therefore only flow through the leak gap between the outer peripheral face of the piston 40 and the inner peripheral face of the cylinder 39. Cutouts 49 are (FIG. 6) formed in the upper end rim of the piston 40. After flowing through the leak gap, the viscous liquid 12 then flows into the region above the piston 40 through the cut-outs 49 with lowering of the piston 40. The displacement of the piston 40 and the plunger 38 can therefore only proceed slowly. As a result, the displacement of the pulley 5 supported on the swinging member 18 can also only proceed slowly under the operation of the damper unit 34, so that the timing belt 2 is controlled by the pulley 5, and growth of oscillators in the timing belt 2 is suppressed.
In the case of the conventional construction shown in FIGS. 3 to 6, the resilient force for pressing the pulley 5 against the timing belt 2 is obtained by the torsion coil spring 28 which is used for that purpose only. Therefore since the biasing spring 41 housed in the damper unit 34 only has the role of extending the damper unit 34, a large resilient force is not required, so that the damper unit 34 can be small. Moreover, since the same type of damper unit 34 can be used for a variety of types of autotensioner having different tension settings, manufacturing costs can be reduced due to simplification of component manufacture and component management.
The construction wherein the spring for applying tension is made independent of the spring for extending the damper unit as mentioned above is well known, and disclosed in other publications such as; Japanese Patent First Publication KOKAI Nos. S58-121344, S61-294249, S61-294250, S62-271910, S62-274142, S62-274143, S62-274144, and H2-89839. Moreover, Japanese Utility Model Publication KOKAI No.H1-100953 discloses an invention wherein as well as the tensioning spring being made independent of the spring for extending the damper unit, the oscillation damping effect is increased by having an oil chamber and reserve chamber in the damper unit which are communicated by a high resistance flow passage.
Through research carried out by the present inventor, it was found that simply having the tensioning spring independent of the spring for extending the damper unit was not sufficient to fully prevent oscillations of the timing belt 2. More specifically, during engine operation, the timing belt 2 is oscillated due to the vibrations transmitted from the engine, and also due to for example fluctuations (size and speed) in the drive force transmitted from the crankshaft, so that the timing belt 2 experiences small changes in tension. That is to say, during engine operation, the tension in the timing belt 2 changes sinusoidally with a frequency proportional to the engine rotational speed.
For a high performance autotensioner, merely applying an appropriate tension to the timing belt 2 by means of a tensioning spring such as the coil spring 28 is not sufficient. The oscillations due to fluctuations in tension must also be adequately suppressed.
On the other hand, with the conventional construction shown for example in FIGS. 3 to 6, when the damper unit 34 is simply made smaller (compared to the first example of the conventional construction shown in FIGS. 1 to 2), the performance of the damper unit 34 becomes inadequate. More specifically, when the tension in the portion of the timing belt 2 controlled by the pulley 5 increases rapidly, then since the overall length of the damper unit 34 is relatively easily contracted, the pulley 5 is relatively easily displaced (in a direction retreating away from the timing belt 2). Hence oscillations in the timing belt 2 cannot be sufficiently suppressed, so that the timing belt 2 tends to flutter.
Also with the invention disclosed in the beforementioned Japanese Utility Model First Publication KOKAI No. H1-100953, although constructed to address the problem related to a sudden drop in tension of the timing belt 2, there is no consideration of the situation as described above for adequate control of the timing belt 2 when the tension increases rapidly.